1. Field of the Invention
This invention relates to an electrodialytic process for the conversion of aqueous solutions of sodium carbonate, sodium bicarbonate, trona or mixtures thereof into sodium hydroxide and carbon dioxide. More specifically this process concerns an electrodialytic process for the conversion of dilute soda sources having anionic and optionally nonionic impurities into high strength, high purity sodium hydroxide and high strength, high purity carbon dioxide gas.
2. Description of the Prior Art
With the discovery of trona ore in the region near Green River, Wyoming, processes useful for the recovery of soda ash products from solutions of trona have been developed. Both uncalcined trona values in dilute aqueous streams, e.g., mine waters, and, calcined trona values in aqueous process streams, e.g., pond liquors or cyrstallizer mother liquor purges have been regarded as waste materials because only limited, commercially feasible methods are known for converting these dilute streams into more valuable products. Particularly valuable products are sodium hydroxide and carbon dioxide.
Methods for the production of sodium hydroxide from sodium carbonate (soda ash) are well known in the prior art. For example, the Solvay lime-soda or ammonia-soda process yields dilute solutions of sodium hydroxide and insoluble calcium carbonate by contactng calcium hydroxide with a soda feed, e.g., sodium carbonate or sodium bicarbonate. However, soluble impurities in the sodium carbonate feed inherently carry through to the dilute sodium hydroxide product, unless they are precipitated with the calcium carbonate or salted out during the evaporative process required to produce commercial strength product. In addition, the amount of lime or calcium hydroxide required must be increased when sodium bicarbonate is used as the soda feed.
The electrolysis of aqueous sodium chloride solutions in cells divided into three compartments by permselective membranes is disclosed in U.S. Pat. Nos. 3,220,941 (Osborne) and 3,438,879 (Kircher et al.). Although these two patents describe methods for the production and separate recovery of hydrogen gas, chlorine gas, sodium hydroxide and sodium carbonate, the amount of chlorine is approximately equivalent to the total alkali produced in the form of sodium hydroxide and sodium carbonate. In addition, the sodium carbonate is generated by contacting the product sodium hydroxide with an external source of carbon dioxide.
Electrodialytic water-splitting processes have been employed in the prior art to recover valuable products from dilute soda streams. For example, U.S. Pat. No. 4,082,835 (Chlanda et al.) discloses an electrodialytic process which utilizes two or three compartment water-splitters to remove SO.sub.2 from dilute gas streams by means of (a) alkaline solution scrubbing, (b) regeneration of the scrubbing solution and, (c) liberation of concentrated SO.sub.2. Although one or more of these operations may be effected in a two compartment water-splitter, the compositions of the alkaline solutions fed to both compartments are identical. Further, the basic product solutions, e.g., aqueous NaOH, NaHCO.sub.3, NaHSO.sub.4 are dilute and impure and, as such are recycled for further alkaline scrubbing of a SO.sub.2 -containing solution.